First, the wiki is your friend, 99% of the time the answer to any question can be found there. That remaining 1% of the time is usually something specific with the game mechanics that isn't as clear as it could be in the wiki.
Second, as a new character with many questions, try to keep the questions to one thread. Not only does this cut down on clutter in the forum, but it gets more people looking at your questions.
Third, the walkthroughs help out alot. They cover some of the basics, like mining, fortress layouts, and job management. I went through half of CaptnDuck's youtube Dwarf Fortress Tutorial prior to starting my own.
Finally. To work with magma, you need magma safe materials (you can use the wiki to find out what they are) this will allow you to pump and stop magma as you need it. You can work with magma without using magma safe materials, but it is both complicated and full of FUN should something go wrong. Flooding the map with magma is inefficent. The magma itself moves too slowly to make it worth while, and if you do flood the map with magma, it will take a bit for it to evaporate. A better idea would be to set up a large trap in which you lure your enemies into it (typically by leaving a path to your front door open). When the enemy is within your trap, close up the exits, and pour magma into the trap itself. Its easier, faster, more efficent, and less cumbersome than flooding the map.
bonus points if you dump water on the magma turning it into obsidian and then carving out the enemy as statues.
+bonus points if you add crafted statues and make the trap your statue garden.